Just like the Crunchy bars you can buy.  If you have time you could make the cinder toffee yourself.

Ingredients: for 12 cupcakes
150g soft butter or margarine
150g caster sugar
3 eggs
150g self raising flour
Finely grated zest of 2 oranges

For the frosting:
100g white vegetable fat such as Trex
400g icing sugar
freshly squeezed orange juice
4 Crunchy bars

1. Preheat the oven to 180c/fan 160c/gas 4.
2. Cream the butter and sugar until pale and fluffy.
3. Add the eggs and flour and mix well.  Stir in the orange zest.
4.  Split the mixture evenly between 12 cupcake cases in a muffin tin and bake for 20-25 minutes.  Leave to cool.
5. To make up the frosting: beat the white vegetable fat in an electric mixer for 20-30 seconds until it’s softened, sift in the icing sugar and add 1 tablespoon of orange juice, slowly start mixing until the icing sugar is amalgamated adding more juice if the icing looks crumbly.  Turn up the mixer to the highest setting and beat for 4-5 minutes until the icing has doubled in volume, it should feel soft when stirred, if it is difficult to stir add a little more water and beat again for 30 seconds.
6. Keep the Crunchy bars in their wrappers and gently hit them with a rolling pin to break them into crumbs, open the wrappers and stir into the frosting, pile onto the tops of the cupcakes and finish with a few Crunchy bar crumbs.